New Tests for Safer Reactors

Wednesday, March 28, 2012 @ 04:03 PM gHale

Operating a nuclear reactor causes progressive microstructural changes in the alloys used in cladding, and that can hurt the materials’ integrity. However, present-day methods of evaluating materials can take decades.

That is why Sandia National Laboratories is using its Ion Beam Laboratory (IBL) to study how to rapidly evaluate tougher advanced materials needed to build the next generation of nuclear reactors and extend the lives of current reactors.

Reactor operators need advanced cladding materials, which are the alloys that create the outer layer of nuclear fuel rods to keep them separate from the cooling fluid. Better alloys will be less likely to deteriorate from exposure to everything from coolant fluids to radiation damage.

The IBL, which replaced an earlier facility dating from the 1970s, has been in operation for about a year and is doing in situ ion irradiation experiments, potentially shaving years off testing. The ion beams use various refractory elements to simulate different types of damage and thus predict the lifetimes of advanced reactor claddings.

Researchers, trying to understand the changes as a function of radiation dose, inserted a beamline from the tandem accelerator, the IBL’s largest, into a transmission electron microscope (TEM). This allowed them to do in situ ion irradiation experiments at the nanoscale and record results rapidly and in real time. Sandia’s lab is one of two facilities in the U.S. and one of only about 10 in the world that can do this, said Khalid Hattar, a materials scientist at Sandia.

“The idea is to come up with new ways to make different alloy compositions and different materials for next-generation reactors and to understand the materials used in the current-generation reactors,” he said. “Then we can find ways of doing a combination of TEM characterization as well as small-scale mechanical property testing in this rapid testing scenario to screen these materials and see which ones are the most suitable.”

Better understanding of cladding materials could help improve reactor efficiency.

Hattar and his team are using the IBL’s capabilities to try to gain a fundamental understanding of how the materials evolve in extreme environments at the nanoscale. They hope that understanding can then relate to events on the macroscale.

Along those lines, take something familiar like rust on a little red wagon.

“If you look at rust, it’s nonuniform,” Hattar said. “So the location where that first rust starts to occur must be related to some heterogeneous aspect of the microstructure. If we can really understand on the nanoscale what causes it, that initiating factor, then we can prevent the initiation, and without the initiation, you’ll never have that rust formation.”

The team developed a system for testing cladding materials that Hattar believes can work in experiments under extreme conditions to simulate real-life environments. Researchers can work with temperatures up to 1,200 degrees C (2,192 degrees F) and pressure up to one atmosphere as well as ion irradiation to gain basic understanding of radiation damage.

A recently completed Laboratory Directed Research & Development (LDRD) program worked with a variety of samples, everything from high-purity, single-crystal copper to materials used in today’s reactors. The team found under the right conditions, a combinatorial approach can work with new alloy compositions produced in-house, Hattar said.

The LDRD project demonstrated a fundamental physics simulation of what’s happening to the material. In the next step, Hattar suggested Idaho National Laboratory expose selected materials to neutrons and then try them out in a real reactor. Since the IBL can run experiments in as little as a day, researchers aim to pinpoint the best material so the Idaho lab, whose tests take much longer, won’t waste time testing poorer materials, he said.

In one experiment, the team examined the composition of and effects of radiation on an alloy considered for the next generation of reactors, seeking the best composition for different radiation exposures.

“Really understanding how the microstructure evolves lets us know a lot about how the material will perform,” Hattar said. “So if we can rapidly determine how the microstructure evolves and understand the mechanisms that it evolves by, we could gain a lot of insight into what happens in the material.”